In recent years, a light source which emits rays of light of blue, green, and red which are three primary colors, each ray of light having an optical output of several watts, has been required with increase in the sizes of display devices. By the way, semiconductor laser diode elements which can emit a ray of blue or green light having a high power of several watts will not be able to be implemented in the near future. Therefore, the most realistic method of providing a ray of blue or green light having a high power of several watts is a method of performing wavelength changing on a high-power fundamental wave produced by a solid state laser device using a nonlinear optical crystal (referred to as a wavelength changing element from here on) having a periodic polarization reversing structure in a state of quasi phase matching. However, these days, because the cost of such a solid state laser device which emits light having a high power is very expensive, a wavelength converter which converts a fundamental wave produced by a semiconductor laser diode element directly into a second harmonic wave using a wavelength changing element is considered to be promising, and therefore research and development of a high-power fundamental-wave light source and a wavelength converter using this light source have been continued.
An example of prior art wavelength converters which uses a monolithic integrated MOPA (an abbreviation of Master Oscillator Power Amplifier) element as a fundamental-wave light source, and which generates a second harmonic wave of about 100 mW from a fundamental wave having an optical output of about 1 W using a wavelength changing element is disclosed. In this MOPA element, a master oscillator having a DBR (Distributed Bragg Reflector) structure which outputs light in a single transverse mode and light in a single longitudinal mode, and a flared semiconductor light amplifier which amplifies the light beams while maintaining their beam quality are integrated on the same semiconductor substrate. As applications of MOPA elements, a combination of a MOPA element and a wavelength changing element provided with a multimode optical waveguide and an example of structure in which a diffraction grating is placed on a rear face of a tapered semiconductor light amplifier are also disclosed (for example, refer to patent reference 1).
Furthermore, there has been disclosed a prior art wavelength converter including a master oscillator in which a resonator in which a diffraction grating is placed at the back of a semiconductor laser diode element is formed, the master oscillator oscillating in a single longitudinal mode, a first optical isolator which prevents light from returning to the master oscillator, a tapered semiconductor light amplifier, and a second optical isolator which prevents light from returning to the semiconductor light amplifier, the prior art wavelength converter generating a stable optical output (for example, refer to patent reference 2).
In addition, there has been disclosed a prior art wavelength converter including a resonator in which a diffraction grating is placed in the front of a semiconductor laser diode element, a master oscillator which oscillates in an injection locking mode by RF-modulating the above-mentioned semiconductor laser diode element, and a tapered semiconductor light amplifier which is a stage located behind the master oscillator, the prior art wavelength converter emitting light having a pulse-shaped waveform (for example, refer to patent reference 3).
Furthermore, there have been disclosed some prior art wavelength converters in which a gain medium in which the reflectivity of a front surface of a semiconductor laser diode is reduced is formed, an external resonator is constructed of a Bragg diffraction grating (referred to as an optical fiber grating from here on) formed in an optical fiber and the semiconductor laser diode element, and the semiconductor laser diode element is made to oscillate in a single longitudinal mode. In addition, there has been disclosed a structure in which light emitted out of such a prior art wavelength converter is optically coupled to a wavelength changing element. Furthermore, there has been disclosed a structure in which the single transverse mode of a semiconductor laser diode is maintained by using a polarization-maintaining optical fiber (for example, refer to patent reference 4).    [Patent reference 1] U.S. Pat. No. 5,321,718 (see the 6th to 12th columns, and FIGS. 5, 7 to 8, and 10)    [Patent reference 2] U.S. Pat. No. 5,745,284 (see the 3rd to 8th columns and FIG. 1)    [Patent reference 3] U.S. Pat. No. 5,561,676 (see the 3rd to 5th columns and FIGS. 1, and 7 to 10)    [Patent reference 4] Japanese patent application laid-open disclosure No. 11-509933,A (see pp. 2 to 12 and FIG. 1)
However, for example, IEEE Photonics Technology Letters (Vol. 10, No. 4, pp. 504-506, 1998) discloses that a monolithic integrated MOPA element as disclosed in above-mentioned patent reference 1 has a problem that its spectrum and optical output vary due to a slight amount of externally reflected light, and also discloses a method of solving the problem. Especially, in a case in which such a monolithic integrated MOPA element is applied to a wavelength changing element, because remaining of slight variations in the spectrum causes a variation in the phase matching with the wavelength changing element and this results in a variation in the wavelength conversion efficiency of the wavelength changing element, it is necessary to insert an optical isolator into the output of the MOPA element so as to remove the externally reflected light. Another problem is however that such an optical isolator which is so designed as not to get optically damaged even if it accepts a fundamental wave of the order of watts is very expensive.
A wavelength converter as disclosed in, for example, patent reference 2, has an advantage of being able to use a cheap and general optical isolator because light having a small power from a master oscillator is simply inputted to the optical isolator. However, a problem with semiconductor light amplifiers is that, as disclosed in, for example, Photonics Technology Letters, Vol. 7, No. 5, and pp. 470-472, a slight amount of return light from outside causes variations in the light distribution in a plane horizontal to the active layer, and therefore an expensive optical isolator which prevents return light is also needed at the outputs of the semiconductor light amplifiers.
In addition, in, for example, the wavelength converter disclosed in patent reference 3, because the resonator consists of a semiconductor laser diode element and a diffraction grating, and a proper high-frequency signal is applied to the semiconductor laser diode element so that it oscillates in an injection locking mode, while the wavelength converter has an advantage of eliminating the necessity to have an expensive optical isolator, a high-frequency generating circuit is needed, a time domain pulse optical output is generated, but no continuous wave is acquired.
Furthermore, the semiconductor laser diode element disposed in the wavelength converter shown in patent reference 4 has a width limit of about 4 micrometers in order to maintain the single transverse mode in the optical waveguide having the active layer therein. Therefore, while a measure of providing a window structure or the like so as to reduce the optical density is taken in order to prevent an optical damage to the emergence end surface of the semiconductor laser diode element, the optical output is quite small, the frequency of chance failures resulting from optical damage caused by defects which occur in the end surfaces and active layer of the semiconductor laser diode element increases when the optical output from the semiconductor laser diode element exceeds 1 W, and the life of the semiconductor laser diode element is shortened remarkably. In addition, in this conventional example, an antireflection film is formed on the front surface of the semiconductor laser diode element so that the semiconductor laser diode element serves as a gain medium, the resonator is constructed of a diffraction grating (referred to as an optical fiber grating from here on) disposed in the optical fiber, and the semiconductor laser diode element, and the semiconductor laser diode element is made to oscillate in the single longitudinal mode. Therefore, another problem is that the spectrum varies again if return light from outside the resonator is incident upon the resonator.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to a fundamental-wave light source which reduces variations in the optical output thereof and variations in the spectrum of the optical output which are caused by external reflection to a master oscillator thereof, and which reduces variations in the light distribution of a semiconductor light amplifier in a lateral direction which are caused by the external reflection, and a wavelength converter light source which can carry out wavelength changing efficiently using the wavelength changing element, and which can output a high-power continuous wave in a short wavelength range.